Merge #14941: rpc: Make unloadwallet wait for complete wallet unload

645e905c327411555073fa7964b36f652998059f doc: Add release notes for unloadwallet change to synchronous call (João Barbosa)
c37851de5752f107c16e19317f28038b6b7ca2dc rpc: Make unloadwallet wait for complete wallet unload (João Barbosa)

Pull request description:

  Currently the `unloadwallet` RPC is asynchronous, it only signals the intent to unload the wallet and then returns the response to the client. The actual unload can happen later and the client has no way to be notified of that.

  This PR makes the `unloadwallet` RPC synchronous, meaning that it blocks until the wallet is fully unloaded.

  Replaces #14919, fixes #14917.

Tree-SHA512: ad88b980e2f3652809a58f904afbfe020299f3aa6a517f495ba943b8d54d4520f6e70074d6749be8f5967065c0f476e0faedcde64c8b4899e5f99c70f0fd6534

5 files changed, 56 insertions, 12 deletions

diff --git a/src/init.cpp b/src/init.cpp
index fa081f8ca1..679bf80047 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -261,10 +261,10 @@ void Shutdown(InitInterfaces& interfaces)
         LogPrintf("%s: Unable to remove pidfile: %s\n", __func__, e.what());
     }
 #endif
+    interfaces.chain_clients.clear();
     UnregisterAllValidationInterfaces();
     GetMainSignals().UnregisterBackgroundSignalScheduler();
     GetMainSignals().UnregisterWithMempoolSignals(mempool);
-    interfaces.chain_clients.clear();
     globalVerifyHandle.reset();
     ECC_Stop();
     LogPrintf("%s: done\n", __func__);
diff --git a/src/wallet/init.cpp b/src/wallet/init.cpp
index 14d811c6cd..87cd264c3d 100644
--- a/src/wallet/init.cpp
+++ b/src/wallet/init.cpp
@@ -242,7 +242,11 @@ void StopWallets()
 
 void UnloadWallets()
 {
-    for (const std::shared_ptr<CWallet>& pwallet : GetWallets()) {
-        RemoveWallet(pwallet);
+    auto wallets = GetWallets();
+    while (!wallets.empty()) {
+        auto wallet = wallets.back();
+        wallets.pop_back();
+        RemoveWallet(wallet);
+        UnloadWallet(std::move(wallet));
     }
 }
diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 38397a3940..39c17743ec 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2637,16 +2637,8 @@ static UniValue unloadwallet(const JSONRPCRequest& request)
     if (!RemoveWallet(wallet)) {
         throw JSONRPCError(RPC_MISC_ERROR, "Requested wallet already unloaded");
     }
-    UnregisterValidationInterface(wallet.get());
 
-    // The wallet can be in use so it's not possible to explicitly unload here.
-    // Just notify the unload intent so that all shared pointers are released.
-    // The wallet will be destroyed once the last shared pointer is released.
-    wallet->NotifyUnload();
-
-    // There's no point in waiting for the wallet to unload.
-    // At this point this method should never fail. The unloading could only
-    // fail due to an unexpected error which would cause a process termination.
+    UnloadWallet(std::move(wallet));
 
     return NullUniValue;
 }
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 536429aeac..098055673b 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -82,13 +82,52 @@ std::shared_ptr<CWallet> GetWallet(const std::string& name)
     return nullptr;
 }
 
+static Mutex g_wallet_release_mutex;
+static std::condition_variable g_wallet_release_cv;
+static std::set<CWallet*> g_unloading_wallet_set;
+
 // Custom deleter for shared_ptr<CWallet>.
 static void ReleaseWallet(CWallet* wallet)
 {
+    // Unregister and delete the wallet right after BlockUntilSyncedToCurrentChain
+    // so that it's in sync with the current chainstate.
     wallet->WalletLogPrintf("Releasing wallet\n");
     wallet->BlockUntilSyncedToCurrentChain();
     wallet->Flush();
+    UnregisterValidationInterface(wallet);
     delete wallet;
+    // Wallet is now released, notify UnloadWallet, if any.
+    {
+        LOCK(g_wallet_release_mutex);
+        if (g_unloading_wallet_set.erase(wallet) == 0) {
+            // UnloadWallet was not called for this wallet, all done.
+            return;
+        }
+    }
+    g_wallet_release_cv.notify_all();
+}
+
+void UnloadWallet(std::shared_ptr<CWallet>&& wallet)
+{
+    // Mark wallet for unloading.
+    CWallet* pwallet = wallet.get();
+    {
+        LOCK(g_wallet_release_mutex);
+        auto it = g_unloading_wallet_set.insert(pwallet);
+        assert(it.second);
+    }
+    // The wallet can be in use so it's not possible to explicitly unload here.
+    // Notify the unload intent so that all remaining shared pointers are
+    // released.
+    pwallet->NotifyUnload();
+    // Time to ditch our shared_ptr and wait for ReleaseWallet call.
+    wallet.reset();
+    {
+        WAIT_LOCK(g_wallet_release_mutex, lock);
+        while (g_unloading_wallet_set.count(pwallet) == 1) {
+            g_wallet_release_cv.wait(lock);
+        }
+    }
 }
 
 const uint32_t BIP32_HARDENED_KEY_LIMIT = 0x80000000;
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index 3a3ec43c9b..6872fbad2d 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -54,6 +54,13 @@ void StopWallets();
 //! Close all wallets.
 void UnloadWallets();
 
+//! Explicitly unload and delete the wallet.
+//  Blocks the current thread after signaling the unload intent so that all
+//  wallet clients release the wallet.
+//  Note that, when blocking is not required, the wallet is implicitly unloaded
+//  by the shared pointer deleter.
+void UnloadWallet(std::shared_ptr<CWallet>&& wallet);
+
 bool AddWallet(const std::shared_ptr<CWallet>& wallet);
 bool RemoveWallet(const std::shared_ptr<CWallet>& wallet);
 bool HasWallets();
@@ -770,6 +777,8 @@ public:
 
     ~CWallet()
     {
+        // Should not have slots connected at this point.
+        assert(NotifyUnload.empty());
         delete encrypted_batch;
         encrypted_batch = nullptr;
     }